1. Field of the Invention
This invention relates to an improved rhodium-organophosphite complex catalyzed hydroformylation process.
2. Background Art
It is well known in the art organophosphites may be employed as catalyst ligands for rhodium based hydroformylation catalysts and that such catalysts exhibit exceptional activity and regioselectivity for producing aldehydes via olefin hydroformylation. For instance, U.S. Pat. Nos. 4,668,651 and 4,769,498 fully detail such hydroformylation.
However, despite the benefits attendant with such rhodium-organophosphite complex catalyzed hydroformylation processes, stability of the ligand and catalyst remains a primary concern. For example, U.S. Pat. No. 4,774,361 is directed to the use of an organic polymer additive to minimize or prevent the rhodium of a rhodium-diorganophosphite complex catalyst from precipitating from solution during the hydroformylation process.
Moreover, over the course of time, continuous rhodium complex catalyzed hydroformylation processes involving organophosphite ligands produce an undesirable hydroxy alkyl phosphonic acid by-product due to reaction of the organophosphite ligand and aldehyde product thereby causing a loss of ligand concentration. Moreover, the formation of such undesirable acid has been observed to be autocatalytic. The build-up of such acid over time could lead to the precipitation of an insoluble gellatinous by-product, which may plug and/or foul the recycle lines of a continuous reaction system, thus necessitating possible periodic processing shut downs or stoppages to remove such acid and/or precipitate from the system by an appropriate method, e.g., by extraction of the acid with a weak base, e.g., sodium bicarbonate. Alternatively, such a problem may be controlled by passing the liquid reaction effluent stream of the continuous liquid recycle process, either prior to or more preferably, after separation of the aldehyde product therefrom, through a weakly basic anion exchange resin bed, as disclosed, e.g., in U.S. Pat. Nos. 4,668,651; 4,717,775 and 4,769,498. A newer and more preferred method for minimizing and/or controlling the problem of such undesirable hydroxy alkyl phosphonic acid by-product resides in the employment of certain epoxide reagents to scavenge such acids, as described, e.g., in assignee's copending U.S. patent application Ser. No. 953,015 entitled "Process For Stabilizing Phosphite Ligands", filed concurrently with this present application.
An additional new problem has been observed when certain organobisphosphite ligand promoted rhodium catalysts are employed in such hydroformylation processes. This problem involves a loss in catalytic activity over time during the course of continuous use of such rhodium-bisphosphite complex catalysts, which loss can occur even in the absence of extrinsic poisons, such as chloride or sulfur compounds.
This intrinsic loss in catalytic activity observed when such organobisphosphite ligand promoted rhodium catalyst systems are employed is believed primarily or at least partly due to the formation of a class of diorganophosphite by-products which can best be described as decomposition products of the employed organobisphosphite ligands so employed. This class of diorganophosphite by-products consists of alkyl [1,1'-biaryl-2,2'-diyl] phosphites, the alkyl radical corresponding to the particular n-aldehyde produced by the hydroformylation process and the [1,1'-biaryl-2,2'-diyl] portion of the phosphite being derived from the organobisphosphite ligand employed. For example, the organobisphosphite ligand shown employed in the continuous hydroformylation process of propylene in Example 14 of U.S. Pat. No. 4,769,498 (referred to as a polyphosphite ligand in said Example 14) will in time, over the course of the continuous hydroformylation process experience an intrinsic decrease in catalytic activity because of the formation of n-butyl [1,1'-biphenyl-2,2'-diyl] phosphite. Such types of alkyl [1,1'-biaryl-2,2'-diyl] phosphites can coordinate with the rhodium metal and form complexes that are less reactive than the preferred organobisphosphite ligand promoted rhodium catalysts. In effect, such types of alkyl [1,1'-biaryl-2,2'-diyl] phosphites so derived act as a catalyst poison or inhibitor, thereby lowering the catalyst activity of the preferred organobisphosphite ligand promoted rhodium catalyst system. More simply, the loss in catalyst activity is a result of the rhodium metal not being utilized to its full potential.